Long wavelength vertical cavity surface emitting laser

ABSTRACT

Selectively oxidized vertical cavity lasers emitting near 1300 nm using InGaAsN quantum wells are reported for the first time which operate continuous wave below, at and above room temperature. The lasers employ two n-type Al 0.94 Ga 0.06 As/GaAs distributed Bragg reflectors each with a selectively oxidized current aperture adjacent to the active region, and the top output mirror contains a tunnel junction to inject holes into the active region. Continuous wave single mode lasing is observed up to 55° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication 60/208,452, filed May 31, 2000, the contents of which arehereby incorporated by reference.

This invention was made with Government support under Contract No.DE-AC04-94AL85000 awarded by the U.S. Department of Energy. TheGovernment has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates generally to vertical cavity surfaceemitting lasers (“VCSEL”), and more particularly, to a VCSEL that emitslight at a nominal wavelength of 1300 nm or higher.

BACKGROUND

Vertical cavity surface emitting laser (VCSEL) sources emitting at 850nm have been widely and rapidly adopted into Gigabit Ethernet and otherapplications. Short wavelength VCSELs are particularly suitable formulti-mode optical fiber local area networks due to their reliability,reduced threshold current, circular output beam, and inexpensive andhigh volume manufacture. However, there is strong interest in developingVCSELs that emit at long wavelengths, such as in the 1240 nm to 1600 nmregime. VCSELs that emit at 1300 nm, for example, may be used toleverage high bandwidth single mode fiber that is often alreadyinstalled as well as to operate at the dispersion minimum of silicaoptical fiber.

SUMMARY OF THE INVENTION

A preferred embodiment of the present invention provides a continuouswave VCSEL that emits at a nominal wavelength of 1300 nm below, at andabove room temperature. In a preferred embodiment, the VCSEL includesInGaAsN/GaAs quantum wells. The laser preferably employs one or moren-type distributed Bragg reflectors and one or more current constrictionapertures adjacent to the optical cavity. The top output mirrorpreferably contains a semiconductor tunnel junction, intracavity contactor other suitable means to inject holes into the active region. Thestructure preferably reduces resistance and optical losses by reducingthe amount of p-type material and placing relatively higher p-typedopant concentrations near standing wave nulls.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings, in which:

FIG. 1 is a cross-sectional view of an exemplary VCSEL structureaccording to the present invention;

FIG. 2 displays structural details of the exemplary VCSEL with astanding wave pattern of the optical field overlaid with the preferredVCSEL layers;

FIG. 2A is a diagram of a tunnel diode positioned at a standing wavenull of the optical field in accordance with an exemplary embodiment ofthe present invention;

FIG. 2B illustrates an upper oxidation layer with a carbon doping spikeat the upper interface in a null in the standing wave pattern;

FIG. 2C illustrates an exemplary mirror design for the VCSEL accordingto the present invention;

FIG. 3 graphically depicts the room temperature output characteristicsof a representative VCSEL with 4.5×4.5 μm² oxide apertures;

FIG. 4 graphically depicts the corresponding single transverse modelasing spectrum at 1294 nm with 28 dB side mode suppression;

FIG. 5A graphically depicts that PL intensity of the InGaAsN quantumwell remains relatively stable with substrate growth temperature, sothat growth of the InGaAsN quantum well is preferably optimized with thesubstrate temperature, T_(SUB), determined by the photo-luminescencewavelength λ_(PL), instead of the photoluminescence intensity I_(PL);

FIG. 5B graphically depicts that the PL wavelength comprises atransition region where the wavelength decreases with decreasingtemperature; and

FIG. 5C graphically depicts the PL intensity versus plasma RF power,P_(RF) with the plasma RF power preferably being optimized to provide ahigh level of photoluminescence intensity I_(PL).

DETAILED DESCRIPTION OF THE INVENTION

An exemplary embodiment of the present invention provides a continuouswave light emitting device that emits at a nominal wavelength of 1300 nmbelow, at and above room temperature. Referring to FIG. 1, an exemplarylight emitting device 10 is a layered structure in which lasing light isemitted in a vertical direction, that is perpendicular to the planes ofthe layers. An exemplary light-emitting device 10 according to thepresent invention may be formed from III-V or II-VI compoundsemiconductor materials with embodiments of the invention preferablybeing in the form of a vertical-cavity surface-emitting laser (VCSEL).The VCSEL preferably exhibits continuous wave single mode lasing up to,at or above 55° C.

Conventional VCSEL designs utilize a thin active region, typically onthe order of one wavelength of the emitted light, to achieve a lowthreshold current. However, such thin active regions have a single passoptical gain of approximately 1%, so that upper and lower mirrors havingreflectivities greater than 99% are typically included to achievelasing. Conventional VCSEL designs typically utilize upper and lowerepitaxially-grown semiconductor distributed Bragg reflector (DBR)mirrors to achieve the required reflectivities. The upper and lowermirrors may be doped with appropriate dopants to have oppositeconductivity types so that the lower mirror, the active region and theupper mirror form a p-i-n structure. A unique challenge for longwavelength VCSELs relative to 850 nm VCSELs is that the opticalabsorption of the p-type doping required for a p-type DBR mirror may beas much as ten times higher in the 1240-1600 nm range.

In a first embodiment of the present invention, the semiconductorlight-emitting device 10 comprises a plurality of compound semiconductorlayers epitaxially-grown on a semiconductor substrate 12. Thesemiconductor layers preferably form a lower mirror stack 14 above thesubstrate, an upper mirror stack 16 above the lower mirror stack, anactive region 18 sandwiched between the mirror stacks, and upper andlower oxidation aperture 22 and 20 respectively located between theactive region and the upper and lower mirror stacks. The oxidationapertures 20 and 22 are also referred to hereinafter as oxide aperturelayers or control layers. The semiconductor layers are etched downwardat least to the lower oxidation aperture 20 as shown in FIG. 1, therebyforming a mesa. An upper electrode 24 is deposited above the uppermirror stack 16; and a lower electrode 26 (also known as a contact) maybe deposited below the substrate 12. An optional insulation layer (notshown) may be provided to protect the etched mesa, and to providesupport for the upper electrode 24.

In an exemplary embodiment, the upper and lower mirror stacks arepreferably n-type distributed Bragg reflector mirrors. The n-typemirrors 14 and 16 are preferably composed of one quarter wavelengththick alternating layers of AlGaAs and GaAs. The utilization of tworelatively low doped n-type mirrors advantageously reduces the freecarrier absorption, which may be excessive at long wavelengths in p-typematerials. A preferred embodiment of the invention incorporates asemiconductor tunnel junction 28 into the high index GaAs layer nearestto the active region 18 in the upper mirror layers 16 to accommodateunipolar n-type mirrors. The heavily doped tunnel junction 28 ispreferably positioned at a node (i.e. a minimum) of the longitudinalfield to reduce or minimize absorption while enabling the conversion ofelectrons to holes for injection into the active region. The oxidationapertures 20 and 22 may be, for example, low index layers of AlGaAswhich are selectively oxidized in part to provide electrical and opticalconfinement.

An exemplary VCSEL 10 may be grown on a GaAs substrate 12, preferablyusing molecular beam epitaxy or other suitable methods. The lower mirror14 is grown above the substrate and is a unipolar structure, having adoping type that is preferably of the same polarity as the substrate 12.The lower oxidation layer 20 is epitaxially grown on the lower mirror 14with an active region 18 grown above the lower oxidation layer 20 andmirror 14. The active region 18 preferably comprises at least one activelayer 30 sandwiched between a pair of barrier layers 32. The upperoxidation layer 22 is grown on the upper barrier layer 32 a and thetunnel junction 28 is formed in a high index GaAs layer nearest to theoptical cavity in the upper mirror layers. The upper mirror layers 16are then grown on the upper surface of the tunnel junction 28.

To electrically contact the VCSEL, upper and lower electrodes 24 and 26are preferably deposited above the upper mirror 16 and below the activeregion 18 respectively. The upper electrode 24 may be deposited beforeor after formation of the mesa. If the upper electrode 24 is depositedbefore the formation of the mesa and patterned by an etching or lift offstep, the upper electrode may either be protected by an overlying layerdeposited and patterned for use as an etch mask, or the upper electrodemay form at least a part of the etch mask. In one embodiment the upperelectrode 24 may be formed, for example, by depositing an n-typemetallization such as AuGe/Ni/Au above the mirror stack 16, and definingan annular opening therein by a lithographic masking and lift-offprocess. Likewise, the lower electrode 26 comprises an n-typemetallization such as AuGe/Ni/Au provided either full-surfacemetallization on the lower surface of the n-type substrate 12, orpatterned to provide an annular aperture therethrough centered about theactive region 18. One of skill in the art will appreciate that formanufacturability, packaging or other purposes two top side electrodesor contacts may be used to electrically contact the VCSEL above andbelow the active region 18.

In an exemplary embodiment, the annular opening formed through the upperelectrode 24 is generally sized to be at least as large in diameter asthe oxide-free portion of the upper oxide layer 22, but smaller indiameter than the top surface of the mesa. In this way, light may beefficiently coupled out from the light-emitting device 10 through thecentral annular opening while allowing the electrical current to beefficiently coupled from the upper electrode 24 into the upper mirrorstack 16, and therefrom to the active region 18. The semiconductorsubstrate 12 is preferably GaAs, that may be doped, for example, n-typewith Si. A semiconductor buffer layer such as a thin layer of GaAs dopedwith Si may initially be epitaxially grown on the substrate 12 prior tothe growth of the lower mirror stack 14. The GaAs substrate 12 istransmissive for wavelengths above 900 nm. Therefore, in some cases,light may be emitted from the lower side of the VCSEL 10 through anannular aperture formed in the lower electrode 26 to enable backsidemonitoring of the laser output intensity for control thereof to maintaina consistent laser output over time.

In the preferred embodiment, the upper and lower mirror stacks are dopedn-type. The n-type mirrors 14 and 16 are preferably composed ofquarter-wavelength-thick alternating layers of Al_(0.94)Ga_(0.06)As andGaAs for operation at a wavelength near 1300 nm. One of skill in the artwill appreciate that the Al fraction of the AlGaAs n-type mirror layersmay vary, preferably in the range of about 0.8-0.96. Further, the upperlimit of the Al fraction may be determined by the Al composition of thealloy used to form the oxide apertures. In the described exemplaryembodiment, the upper and lower mirrors 16 and 14 are preferably highlyreflective, preferably >99% reflectivity, to provide a high qualityfactor, Q, for the resonant optical cavity formed between the mirrorstacks. In addition, the active region 18 is designed to providesufficient optical gain for lasing action in the cavity over a range ofoperating currents. The reflectivities of the upper and lower mirrorstacks 16 and 14 may be defined during epitaxial growth of the lightemitting device 10 by adjusting the number of mirror periods formingeach mirror stack. In addition, the reflectivities of the mirror stacksmay also be adjusted by adjusting the semiconductor alloy compositionsof a high index of refraction semiconductor layer and a low index ofrefraction layer forming each mirror period. In an exemplary embodiment,the upper mirror stack 16 contains 28 periods and the lower mirror stack14 contains 33 periods.

An exemplary design for the upper and lower mirrors 16 and 14 is shownin FIG. 2C. The n-type upper and lower mirrors 16 and 14 are preferablySi doped. The upper and lower mirrors 16 and 14 are designed to reducethe electrical resistance between adjacent semiconductor layers due tooffsets in a conduction band, or a valence band or both that mightotherwise give rise to a voltage drop across the mirrors. For a VCSEL,this voltage drop increases a bias voltage across the device andproduces excess heating therein, thereby deteriorating device efficiencyand performance.

In a preferred embodiment of the present invention, the GaAs high indexof refraction layers may be doped at concentrations in the range ofabout 2×10¹⁷-2×10¹⁸ cm⁻³ with a preferred concentration near 5×10¹⁷cm⁻³. However, as shown in FIG. 2C the AlGaAs low index of refractionlayers also can have increased doping at the layer edges near theadjoining GaAs layers. In a preferred embodiment, the AlGaAs mirrorlayers are preferably Si doped at a concentration of approximately of3×10¹⁸ cm⁻³ for approximately 130 angstroms nearest the layer interfacesto reduce the electrical resistance between adjacent semiconductorlayers. The remainder of each AlGaAs layer may be Si doped with a lowerconcentration in the range of about 5×10¹⁷-4×10¹⁸ cm⁻³ preferably with aconcentration of 1×10¹⁸ cm⁻³. Additionally, the first few periods ofeach mirror stack proximate to the active region 18 may be grown with areduced dopant concentration as compared to the remaining periods. Thereduced dopant concentration reduces the optical loss in the resonantcavity formed by the upper and lower mirrors 16 and 14 due to dopantimpurity absorption and light scattering. Furthermore, the dopantconcentration at the top 100-200 angstroms of the upper mirror stack 16may be increased to about 1×10¹⁹ cm⁻³ or more to facilitate electricallycontacting the upper mirror stack 16 with the upper electrode 24.

The upper and lower oxide apertures 22 and 20 are formed above and belowthe active region 18 respectively. The upper and lower oxide apertures22 and 20 preferably comprise a semiconductor alloy containing aluminum.The semiconductor alloy may be oxidized in part after a mesa is formedin the VCSEL structure, preferably to or below the lower oxide layer.The oxidized outer portion of each of the oxide aperture layers hasincreased resistivity providing lateral current constriction to controltransverse higher order modes. The current constriction formed by theoxidized portion of the oxide aperture layers preferably reduces thediameter or size of the current aperture below the outside diameter orsize formed by the VCSEL electrodes. In addition, an oxide free centralportion of the oxide aperture layers preferably remains transmissive tolight.

In a preferred embodiment of the present invention, the oxidized outerportion of the upper and lower oxide apertures 22, 20 generally have anannular shape with the oxidation extending inward from one or moreetched sidewalls of the mesa. The lateral shape of the annular oxidizedportion will depend upon the shape of the mesa (for example, circular,square, rectangular, or elliptical when viewed in the direction of thelight emitted from the device 10) and the number of sidewalls exposed toan oxidation process. The lateral shape of the annular oxidized portionof the oxide apertures 20, 22 may also be influenced or controlled byadditional factors which may affect the oxidation process including thesemiconductor alloy composition of the oxide apertures 20, 22.Crystallographic preferences for oxidation may also be present, forexample, due to a grading of the semiconductor alloy composition of theoxide apertures 20, 22 in the growth direction, or strain in theepitaxial layers.

The upper and lower oxide apertures 22, 20 comprise one or more oxideaperture layers having a semiconductor alloy composition or layerthickness that is different from the composition and layer thickness ofany of the other compound semiconductor layers. As an example, the oxideaperture layers may be formed from AlAs or preferably from AlGaAs withan aluminum composition higher than the aluminum composition of AlGaAshigh-bandgap semiconductor layers in the upper and lower mirror 16, 14.In this example, it is also preferable that the aluminum composition ofthe oxide layers be higher than any of the layers of the active region18.

The aluminum composition of the oxide aperture layers may be used toselectively oxidize the semiconductor alloy, converting it to an oxideof aluminum. The lateral extent of oxidation of the oxide layersaccording to the present invention is greater than any lateral extent ofoxidation of the other semiconductor layers having exposed edges in themesa. This selective oxidation is due to a strong compositionaldependence in the lateral oxidation of Al_(x)Ga_(1-x)As layers for x inthe range of about 0.8 to 1.0. (The oxidation rate of the semiconductoralloy AlGaAs may also be viewed as being suppressed by the addition ofsmall amounts of gallium to the alloy). Preferably, the oxide layersforming the upper and lower oxide apertures 22, 20 have an aluminumcomposition sufficiently high to provide about a 5:1 or higher increasein the lateral oxidation extent of the oxide layers as compared withother aluminum-containing layers in the mesa.

The oxide apertures 20, 22 are preferably doped with a dopant type thatis the same as the mirror layer or tunnel junction layer immediatelyadjacent to the oxide layer. Thus, the lower oxide aperture 20 that islocated between the active region 18 and the lower mirror stack 14, ispreferably doped n-type with a Si or other suitable dopant. The dopantdensity in the lower oxide aperture 20 may be on the order of about1×10¹⁸ cm⁻³. The upper oxide aperture 22, located between the activeregion 18 and the p-type layer of the tunnel junction, is preferablydoped p-type with a relatively low doping density of Be, C or othersuitable dopant. The dopant density in the upper oxide aperture may bein the range of about 1×10¹⁶-5×10¹⁷ cm⁻³, with a preferred density onthe order of about 2×10¹⁶ cm⁻³. One of skill in the art will appreciatethat the upper oxidation aperture may be doped with other suitablematerials or at higher doping densities with a commensurate increase inthe optical loss.

In the described exemplary embodiment, the layer thickness of theannular oxidized portion of the oxide aperture layers may be slightlydifferent from the layer thickness of the central oxide-free portion(due to a change in the chemical composition of the oxidized portion).However, the effective optical thickness (i.e. the layer thicknessmultiplied by the refractive index, n) of the two portions of the oxideapertures may be significantly different. This large difference in theeffective optical thickness is due to the large difference in therefractive index of the oxidized and oxide-free portions of the oxidelayers. Thus, the oxide apertures 20, 22 may provide a phase shift ofthe light in the optical cavity passing through the annular oxidizedportion of each oxide aperture layers that is substantially differentfrom the phase shift of the light passing through the central oxide-freeportion of the oxide aperture layers. This phase shift is due to thedifference in refractive indices of the two portions of the oxideapertures 20, 22. The thickness for the oxide apertures 20, 22 may bechosen to provide a phase shift in the annular oxidized portion that is,for example, substantially equal to a multiple of one-half of thewavelength of the light generated in the active region 18, while thephase shift in the oxide-free portion is substantially equal to amultiple of one-quarter of the wavelength of the light.

A compound semiconductor active region 18 is epitaxially grownsandwiched between the mirror stacks 14 and 16. The active region 18 hasa thickness that is preferably an integral multiple of one-half of thewavelength of the light generated in the active region. The activeregion 18 may be either undoped (i.e. intrinsic, or not intentionallydoped); or a portion on either side of the active region may be doped toform a semiconductor p-n or p-i-n junction within the active region 18.The active region may include one or more quantum-wells 30 surrounded bybarrier layers as may be preferable for the formation of a VCSEL device10. The quantum-wells provide quantum confinement of electrons and holestherein to enhance recombination for the generation of the light, andmay also include semiconductor layers comprising a plurality of quantumwires or quantum dots therein.

In an exemplary embodiment of the present invention, the active region18 comprises one or more quantum-well layers 30 containing one or moreIn_(x)Ga_(1-x)As_(1-y)N_(y) quantum wells designed to emit atwavelengths in the range of 1240 nm to 1360 nm. Fractionally the In mayrange from about 0.3-0.4, and the Nitrogen may range from about0.01-0.02. In a preferred embodiment, there are twoIn_(0.34)Ga_(0.66)As_(0.99)N_(0.01) quantum wells, with barrier layerssurrounding and separating the quantum wells. The barrier layerspreferably have an energy bandgap intermediate between the energybandgaps of the quantum-well layers 30 and the oxide free portion of theoxidation apertures 20 and 22. In an exemplary embodiment, the opticalcavity is one wavelength thick, (i.e. an effective optical thicknessthat is substantially equal to one wavelength of the lasing lightgenerated by the VCSEL 10). Each quantum well 30 may be in the range ofabout 3-10 nm thick and is preferably on the order of about 6 nm thick.In addition, each barrier layer may be on the order of about 20 nm thickwhen separating a pair of adjacent quantum wells and is otherwise about174 nm thick. The barrier layers may comprise, for example, undopedGaAs.

Alternatively, the optical cavity may further include cladding layerssurrounding the barrier layers, in which the cladding layers have anenergy bandgap equal to that of the barrier layers or intermediatebetween the energy bandgap of the barrier layer and the oxide freeportion of the oxidation layers. In some cases the barrier layers maycomprise the same material as the cladding layer. The cladding layersmay have a semiconductor alloy composition that is uniform in the growthdirection, forming a separate confinement heterostructure (SCH) activeregion 18. Alternately, the semiconductor alloy composition of thecladding layers may be graded in the growth direction (i.e. graded froma higher-bandgap to a lower-bandgap alloy composition for thefirst-grown cladding layer). Similarly, the second grown cladding layermay be graded from a lower-bandgap alloy to a higher-bandgapcomposition, forming a graded-index separate confinement heterostructure(GRIN-SCH) active region 18.

The number and location of quantum-wells in a VCSEL device 10 mayfurther provide means for increasing the optical gain by maximizing aspatial overlap with an electric field antinode (i.e. maximum) of thelight in the resonant optical cavity. The quantum-well layers 30 may bepositioned near an antinode of the electric field of the light in theoptical cavity to increase the efficiency for light generation therein.

The preferred embodiment incorporates a tunnel junction 28 into the highindex GaAs layer nearest to the active region 18 in the upper mirror 16.The tunnel junction 28 injects holes into the active layer withouthaving the absorption that is characteristic of a p-type top outputmirror. The heavily doped tunnel junction 28 is preferably positioned ata node of the longitudinal electric field of the light to reduce orminimize absorption while enabling the conversion of electrons to holesfor injection into the active region. The preferred embodiment includeslow index layers (e.g. AlAs) for the oxide free portions of the oxideapertures 20 and 22 immediately adjacent to each side of the activeregion 18.

The mesa may be formed by etching down at least to the lower oxideaperture by a wet or preferably a dry etching process such as reactiveion etching (RIE), reactive ion beam etching (RIBE), or the like. Themesa is formed by lithographically patterning the top surface of theupper mirror 16 and depositing thereon a suitable material (for example,silicon nitride, silicon oxide, silicon oxynitride, metal silicides, orrefractory metals) as an etch mask. After etching the mesa structuredown to or through the lower oxide aperture 20, the etch mask may beleft in place to protect the top layer of the upper mirror, or removedprior to the oxidation process. In forming the mesa, the etch depth maybe measured in-situ by reflectometry to provide a precise control of theetch depth, and to allow the etch process to be stopped after etchingdown at least to the lower oxide aperture. In some instances, it may bepreferable to etch down beyond the lower oxide aperture to providemore-vertical sidewalls for the mesa for uniform oxidation of one ormore oxide apertures. Furthermore, in other embodiment of the presentinvention, the mesa may be omitted and instead a plurality of trenchesor wells or an annular trench may be etched down through the variouslayers above the active region 18 to permit lateral oxidation to formthe oxide apertures.

The oxidation process may be carried out by placing the wafer into acontainer and heating the wafer to a temperature of about 350 to 500° C.(and preferably between about 400 and 450° C.) under a controlledenvironment having a high humidity. Such a moist environment may begenerated, for example, by flowing a gas, such as nitrogen, throughwater heated to about 80-95° C. to entrain water vapor, and thendirecting the moisture-laden gas into the container.

The time required for formation of the annular oxidized portion of theupper and lower oxide apertures 22, 20 depends upon a number ofvariables. For example, the formation time may vary in accordance withthe aluminum composition of the oxide aperture layers forming the oxideaperture 20 and 22, the temperature to which the semiconductor wafer isheated, the thickness of the oxide aperture layers and the lateralextent to which the oxide aperture layers are to be oxidized (i.e. thelateral dimension of the annular oxidized portion). Generally, a 50 nmthick oxide aperture layer will oxidize in about 30 to 150 minutes withan oxidation temperature in the range of about 400 to 450° C. Thecomposition and quality of the aluminum oxide formed by the oxidationprocess may also be temperature dependent.

After the oxidation process is completed, an insulation layer (notshown) may be deposited onto the semiconductor wafer to protect andpassivate the etched mesa and exposed semiconductor layers, and toplanarize the light-emitting device 10 formed on the semiconductorwafer. The insulation layer may be formed of any insulating material asis commonly used in the art including polyimides, spin-on-glasses,silicon dioxide, silicon nitride, and the like.

FIG. 2 displays structural details of the 1300 nm VCSEL shown in FIG. 1,overlayed with the standing wave intensity profile of the optical fieldas a function of vertical position within the VCSEL. The standing waveintensity profile is related to the intensity of the light in the VCSEL.Hence, the standing wave maxima are where the circulating light in thecavity is most intense, and the standing wave minima are where the lightis least intense. Light is more readily absorbed by high dopedsemiconductor materials and less absorbed by low doped materials.Therefore, in an exemplary VCSEL 10 according to the present invention,the tunnel junction 28 is preferably positioned to place the high-dopedsemiconductor materials therein at a standing wave null in the opticalfield (i.e. at a node). Advantageously, by placing the high-dopedsemiconductor materials at the intensity minima, light absorption in thetunnel junction is minimized.

Referring to FIG. 2A, the tunnel junction 28 preferably comprises ap-type layer and an n-type layer. The p-type layer may be in the rangeof about 10-100 angstroms thick and in an exemplary embodiment is on theorder of 48 angstroms thick. The p-type layer of the tunnel junction maybe doped with a carbon (C) dopant at a density in the range of about2×10¹⁹-2×10²⁰ cm⁻³ and preferably on the order of about 1×10²⁰ cm⁻³. Then-type layer of the tunnel junction may be in the range of 50-500angstroms thick and is preferably on the order of 250 angstroms thick.The n-type layer of the tunnel junction may be doped with a silicon (Si)dopant at a density in the range of about 4×10¹⁸-4×10¹⁹ cm⁻³ and ispreferably on the order of approximately 1.4 ×10¹⁹ cm⁻³.

In an exemplary embodiment of the present invention the oxide aperturelayer forming the upper oxidation aperture is doped p-type, andprimarily consists of relatively low density Be dopant, preferably onthe order of about 2×10¹⁶ cm⁻³. The lower doping density reducesabsorption of photons traversing one or more oxidation aperture layersforming the upper oxide aperture 22. However, as seen in FIG. 2B theupper oxidation aperture 22 preferably includes a carbon doping spike atthe upper interface thereof in a null in the standing wave pattern ofthe optical field. The carbon doping spike is preferably on the order of20 angstroms thick with a p-type carbon doping density of approximately2×10¹⁹ cm⁻³. The doping spike preferably reduces the resistance at theheterointerface while maintaining low optical loss due to its locationat the standing wave null in the optical field.

An exemplary 1300 nm VCSEL with one or more InGaAsN quantum wells 30 ispreferably grown on a GaAs substrate using molecular beam epitaxymanufacturing techniques. The technique preferably optimizes the indiumand nitrogen incorporation and the substrate growth temperature foroptimal quality of the InGaAsN quantum wells. As is known in the art,the actual growth parameters for each epitaxial layer typically varydepending on the particular MBE system used. In one embodiment, theVCSEL may be fabricated using an MBE system with a nitrogen plasmasource for adding nitrogen to the InGaAsN quantum wells 30. In apreferred embodiment, the deposition of the InGaAsN quantum wells 30preferably attempts to increase the concentration of indium and reducethe concentration of nitrogen to obtain a high quality device emittingat a particular wavelength near 1300 nm. Careful attention to growth ofeach InGaAsN quantum well is needed since increasing the indiumconcentration tends to increase the emission wavelength but alsoincreases the stress in the quantum well layers, thereby necessitating areduction in quantum well thickness to avoid stress induceddislocations. However, reducing the well thickness also leads to areduction in the emission wavelength due to increased quantumconfinement. Furthermore, increasing the nitrogen concentration tends toincrease the operating wavelength, and further provides straincompensation for the indium. Adding too much nitrogen, however, leads topoor optical quality of the material. Therefore, in a preferredembodiment, the semiconductor alloy composition of each InGaAsN quantumwell 30 is optimized to achieve 1300 nm emission without exceedingcritical thickness of the quantum wells 30 using a minimum amount ofnitrogen therein. In a preferred embodiment nitrogen fraction in each ofthe InGaAsN quantum wells 30 is at least 0.01 and preferably in therange of 0.01-0.02.

Material concentrations may be optimized by controlling the MBE nitrogenplasma conditions as well as the growth temperature of the substrate 12.The operating parameters of the plasma source that may be controlledinclude the RF power level, aperture size and gas pressure or gas flowrate. FIG. SC graphically depicts the photoluminescence (PL) intensityfrom an InGaAsN quantum well 30 versus plasma RF power. The RF powerlevel is preferably reduced to the lowest reasonable level to provideoptimal PL intensity. The aperture size and input flow are then adjustedto provide the N flux.

Conventionally, those of ordinary skill in the art use the PL intensityto determine the optimal substrate growth temperature for high qualityepitaxial growth of each semiconductor layer with a VCSEL or edgeemitting laser, and especially for the quantum wells. However, it hasbeen found that the PL intensity of the InGaAsN quantum well remainsrelatively stable and independent of substrate growth temperature asshown in FIG. 5A. However, the performance of a particular laser havingan InGaAsN quantum well is highly dependent upon substrate growthtemperature with a narrow optimal range. Therefore, in the case ofInGaAsN, the PL intensity signature does not indicate the optimumsubstrate growth temperature.

However, we have found that the PL wavelength for an InGaAsN quantumwell 30 demonstrates a dependence on the substrate growth temperature asshown in FIG. 5B. Generally, the PL wavelength of InGaAsN may beincreased by increasing the nitrogen or indium content. The inclusion ofboth materials in suitable quantities reduces the strain in the quantumwell. However, one would expect that volatile materials like nitrogenwould incorporate more strongly at lower substrate growth temperaturesthereby lowering the energy bandgap of the InGaAsN. Therefore, one ofskill in the art would expect the PL wavelength to increase withdecreasing temperature.

As shown in FIG. 5B, however, according to the present invention the PLwavelength is shown to generally comprise a transition region where thewavelength decreases with decreasing temperature contrary toexpectation, and outside this transition region the PL wavelength isrelatively independent of the growth temperature. This transition regionis indicative of an increase in the energy bandgap of the InGaAsNquantum well material with decreasing temperature, which suggests thatthe nitrogen may be distributed more homogenously with less phaseseparation or clustering. The preferred substrate growth temperatureaccording to the present invention, that is typically at the lowtemperature edge of the transition region, as indicated by the dashedline in FIG. 5B, is at a temperature on the order of about 415° C. Oncethe location of the narrow growth temperature range of the transitionregion producing the wavelength shift in FIG. 5B has been located usingPL data, it is then preferred to fabricate a plurality of edge emittinglasers emitting at a nominal wavelength of 1300 nm. The edge emittersare preferably similar epitaxial structures grown using differentsubstrate temperatures near the transition region and preferably nearthe low temperature edge of the transition region. The measuredthreshold current characteristics for these edge-emitting lasers maythen be used to determine the optimal growth temperature for the InGaAsNquantum wells 30 in subsequently grown VCSELs 10. The preferred processmethodology enables one to efficiently locate the optimum substrategrowth temperature in a systematic and repeatable manner and in a mannerthat is more simple than growing and testing a plurality of VCSELs atdifferent growth temperatures.

FIG. 3 graphically depicts the room temperature operatingcharacteristics of a representative VCSEL with two square 4.5×4.5 μm²oxide apertures defined by the inside dimension of the oxidized portionof the oxide aperture layers. VCSELs with square apertures varying insize from 2×2 up to 12×12 μm² operated with room temperature thresholdcurrents varying from approximately 2 to 10 mA, with the thresholdcurrent increasing with size of the oxide aperture. The broad areathreshold current density for each of these VCSELs was on the order of 4kA/cm². The representative VCSEL 10 of FIG. 3, is preferably fabricatedby the oxidation method disclosed in U.S. Pat. No. 5,493,577 which isincorporated herein by reference. It is understood that the VCSEL may befabricated by any technique known in the art for fabricating VCSELssubject to the modifications described herein.

FIG. 4 graphically depicts the corresponding single transverse modelasing spectrum at 1294 nm with 28 dB side mode suppression. Single modeoutput power of 60 μW is obtained at 20° C. and continuous wave lasingis observed up to 55° C. The threshold current monotonically decreaseswith decreasing temperature (down to 5° C.), which implies that thelaser gain maximum is at a wavelength longer than 1294 nm. If the gainmaximum is selected to be at a wavelength shorter than the desiredemission wavelength, the laser temperature performance may be furtherimproved. The submilliwatt maximum output in FIG. 3 is the combinedresult of the high reflectivity output coupler and the spectralmisalignment between the cavity resonance and the laser gain profile. Aswill be recognized by one of skill in the art, the relatively highoperating voltage in FIG. 3 may be reduced by incorporatingcompositional mirror grading into the mirror design, such as taught inU.S. Pat. No. 5,568,499, the contents of which are hereby incorporatedby reference. Therefore the disclosed mirror structure is by way ofexample and not by limitation.

VCSEL technology has historically provided the most cost efficientoptical link solution for high bandwidth applications, as demonstratedin recent years by their rapid adoption over edge-emitting lasers in thedata communications market. A VCSEL preferably has a narrower linewidththan a Fabry-Perot laser for SONET, Ethernet, fiber to the homeapplications; may also include a transparent GaAs substrate for backsidemonitoring. The exemplary 1300 nm VCSEL enables extended distances anddata rates to be realized over single mode optical fiber. The exemplary1300 nm VCSEL will therefore provide significant cost reductions makingincreased bandwidth more accessible and cost effective for thetelecommunications and Internet infrastructure. The exemplary 1300 nmVCSEL also may be applied in the fiber channel, Gigabit Ethernet,10-Gigagbit Ethernet, fiber to home markets and any other desiredapplication. Moreover, the exemplary 1300 nm VCSEL may one day renderconventional longwave Fabry-Perot and DFB lasers relatively lessdesirable for many applications.

Although a preferred embodiment of the present invention has beendescribed, it should not be construed to limit the scope of the appendedclaims. Those skilled in the art will understand that variousmodifications may be made to the described embodiment. Moreover, tothose skilled in the various arts, the invention itself herein willsuggest solutions to other tasks and adaptations for other applications.It is therefore desired that the present embodiments be considered inall respects as illustrative and not restrictive, reference being madeto the appended claims rather than the foregoing description to indicatethe scope of the invention.

1. A vertical cavity surface emitting laser comprising: a substrate; a first n-type mirror adjacent the substrate; an active region including one or more quantum wells, the quantum wells being formed of InGaAsN; a second n-type mirror adjacent the active region, the second mirror including a tunnel junction for injecting holes into the active region, wherein the laser emits light at a nominal wavelength of 1300 nm.
 2. The vertical cavity surface emitting laser of claim 1, wherein the substrate includes GaAs.
 3. The vertical cavity surface emitting laser of claim 1, wherein the tunnel junction includes a n-type layer and a p-type layer.
 4. The vertical cavity surface emitting laser of claim 3, wherein the p-type layer of the tunnel junction is positioned at or near a standing wave null in optical field.
 5. The vertical cavity surface emitting laser of claim 2, further comprising one or more oxide apertures, proximate to the active region, wherein the oxide aperture includes an oxidized portion therein.
 6. The vertical cavity surface emitting laser of claim 5 wherein the oxidized portion of the oxide aperture comprises an aluminum oxide.
 7. The vertical cavity surface emitting laser of claim 5 wherein the oxide aperture comprises a carbon doped spike positioned at or near a standing wave null in optical field.
 8. The vertical cavity surface emitting laser of claim 5, further comprising a mesa extending downward at least to the oxide aperture.
 9. The vertical cavity surface emitting laser of claim 1 wherein the first and second n-type mirrors comprise unipolar distributed Bragg reflector mirrors.
 10. The vertical cavity surface emitting laser of claim 1 further comprising an upper electrode above the second mirror stack and a lower electrode below the active region.
 11. The vertical cavity surface emitting laser of claim 10 wherein the lower electrode includes an annular aperture therein to monitor transmitted output power of the vertical cavity surface emitting laser from light emitted through the annular aperture in the lower electrode.
 12. A method of manufacturing a surface emitting laser that emits light at a nominal wavelength of 1300 nm., comprising: forming a first n-type mirror on a substrate; forming an active region having one or more InGaAsN quantum wells on the substrate; forming a current constriction proximate the active region; forming a second n-type mirror above the active region; and forming a tunnel junction in the second n-type mirror, wherein the tunnel junction comprises an n-type region and a p-type region and the p-type region is positioned at or near a standing wave null in optical field.
 13. The method of claim 12 wherein the step of forming a current constriction comprises forming oxide aperture layers proximate to said active region.
 14. The method of claim 13 wherein the step of forming oxide aperture layers proximate to said active region comprises forming at least one aluminum alloy layer proximate to said active region.
 15. The method of claim 14 further comprising forming a mesa downward from upper most surface of the surface emitting laser to the oxide aperture layers and oxidizing an annular portion of said oxide aperture layers.
 16. The method of claim 14 wherein the step of forming oxide aperture layers further comprises doping each aluminum alloy layer with an n-type or p-type dopant.
 17. The method of claim 16 wherein the step of doping the aluminum alloy layer with the p-type dopant further comprises forming a carbon doped spike in said aluminum alloy layer, wherein said carbon doped spike is positioned at or near a standing wave null in the optical field.
 18. The method of claim 12 wherein the step of forming said second mirror comprises forming one or more pairs of semiconductor mirror layers, wherein one layer in each pair has an index of refraction that is different from the index of refraction of the layer in each pair.
 19. The method of claim 18 wherein the step of forming said semiconductor mirror layers comprises forming one quarter wavelength thick alternating layers of AIGaAs and GaAs, wherein said tunnel junction is formed into the GaAs layer nearest said active region.
 20. The method of claim 12 further comprising forming an upper electrode above the second mirror and forming a lower electrode below the active region.
 21. The method of claim 20 wherein the steps of forming the upper and lower electrodes comprises forming at least one of the upper and lower electrodes having an annular aperture therein. 